How will you distinguish between formaldehyde and acetaldehyde?

Answer 1

Using Tollens' Reagent (The Silver Mirror Test):


Tollens' reagent contains the diamminesilver(I) ion, [Ag(NH3)2]+.
This is made from silver(I) nitrate solution. You add a drop of sodium hydroxide solution to give a precipitate of silver(I) oxide, and then add just enough dilute ammonia solution to redissolve the precipitate.
To carry out the test, you add a few drops of the aldehyde or ketone to the freshly prepared reagent, and warm gently in a hot water bath for a few minutes.

• Ketone : No change in the colourless solution.
• Aldehyde : The colourless solution produces a grey precipitate silver, or a silver mirror on the test tube.

Aldehydes reduce the diamminesilver(I) ion to metallic silver. Because the solution is alkaline, the aldehyde itself is oxidised to a salt of the corresponding carboxylic acid.

• 2Ag(NH3)2+ + RCHO + 3OH- ____} 2Ag + RCOO- + 4NH3 + 2H2O

Using Fehling's Solution or Benedict's Solution:
Fehling's solution and Benedict's solution are variants of essentially the same thing. Both contain complexed copper(II) ions in an alkaline solution.
Fehling's solution contains copper(II) ions complexed with tartrate ions in sodium hydroxide solution. Complexing the copper(II) ions with tartrate ions prevents precipitation of copper(II) hydroxide.
Benedict's solution contains copper(II) ions complexed with citrate ions in sodium carbonate solution. Again, complexing the copper(II) ions prevents the formation of a precipitate - this time of copper(II) carbonate.
Both solutions are used in the same way. A few drops of the aldehyde or ketone are added to the reagent, and the mixture is warmed gently in a hot water bath for a few minute

• Ketone: No change in the blue solution
• Aldehyde: The blue solution produces a dark red precipitate of copper(I) oxide.

Aldehydes reduce the complexed copper(II) ion to copper(I) oxide. Because the solution is alkaline, the aldehyde itself is oxidised to a salt of the corresponding carboxylic acid.

• RCHO + NaOH +Cu(OH)2 ____} RCOONa + Cu2O + 3H2O

Using Acidified Potassium Dichromate(VI) Solution:


A small amount of potassium dichromate(VI) solution is acidified with dilute sulphuric acid and a few drops of the aldehyde or ketone are added. If nothing happens in the cold, the mixture is warmed gently for a couple of minutes - for example, in a beaker of hot water. Kketone No change in the orange solution. Aldehyde Orange solution turns green.
The orange dichromate(VI) ions have been reduced to green chromium(III) ions by the aldehyde. In turn the aldehyde is oxidised to the corresponding carboxylic acid.
The electron-half-equation for the reduction of dichromate(VI) ions is:

• 3RCHO + Cr2O7 -2 + 8H+ ____} 3RCOOH + 2Cr+3 + 4H2O

Answer 2

Benzaldehyde gives silver mirror test with Tollen's reagent but not acetophenone.

Answer 3

Formaldehyde is easily oxidised to CO2 and acetaldehyde is can only be oxidised to acid, NOT to CO2